// RESULT: 0.864664716763387 (1-e^-2)
	filter(first, P=? [ "a" U<=2 "b" ], x=1);
	// RESULT: ?
	"phi": ("a" & "b") | P<=0.8 [ "a" U<=2 "b" ];
	// RESULT: true
	filter(forall, "phi" <=> (x=0 | x=3));